New vitamin D receptor ligands

Design, synthesis, and biological activity of D-bishomo-1α,25-dihydroxyvitamin D3 analogs and their crystal structures with the vitamin D nuclear receptor

The biologically active metabolite of vitamin D3 - calcitriol - is a hormone involved in the regulation of calcium-phosphate homeostasis, immunological processes and cell differentiation, being therefore essential for the proper functioning of the human body. This suggests many applications of this steroid in the treatment of diseases such as rickets, psoriasis and some cancers. Unfortunately, using therapeutic doses of calcitriol is associated with high concentrations of this compound which causes hypercalcemia. For this reason, new calcitriol analogs are constantly sought, devoid of calcemic effects but maintaining its beneficial properties. In this study, we present the synthesis of vitamin D derivatives characterized by an enlarged (seven-membered) ring D. Preparation of the designed vitamin D compounds required separate syntheses of crucial building blocks (C/D-rings fragments with side chain and rings A) which were combined by different methods, including Wittig-Horner reaction and Suzuki coupling. Biological activities of the target vitamin D analogs were assessed both in vitro and in vivo, demonstrating their significant potency compared to the natural hormone. Furthermore, the successful crystallization of these compounds with the vitamin D receptor (VDR) enabled us to investigate additional molecular interactions with this protein.

Vitamin D and Its Synthetic Analogs

For many individuals, in particular during winter, supplementation with the secosteroid vitamin D₃ is essential for the prevention of bone disorders, muscle weakness, autoimmune diseases, and possibly also different types of cancer. Vitamin D₃ acts via its metabolite 1α,25-dihydroxyvitamin D₃ [1,25(OH)₂D₃] as potent agonist of the transcription factor vitamin D receptor (VDR). Thus, vitamin D directly affects chromatin structure and gene regulation at thousands of genomic loci, i.e., the epigenome and transcriptome of its target tissues. Modifications of 1,25(OH)₂D₃ at its side-chain, A-ring, triene system, or C-ring, alone and in combination, as well as nonsteroidal mimics provided numerous potent VDR agonists and some antagonists. The nearly 150 crystal structures of VDR’s ligand-binding domain with various vitamin D compounds allow a detailed molecular understanding of their action. This review discusses the most important vitamin D analogs presented during the past 10 years and molecular insight derived from new structural information on the VDR protein.

Synthesis of 1α,25-dihydroxyvitamin D3 analogues with α,α-difluorocycloketone at the CD-ring side chains and their biological properties in ovariectomized rats

Two novel 1α,25-dihydroxyvitamin D₃ derivatives containing a α,α-difluorocyclopentanone (3) or α,α-difluorocyclohexanone (4) moiety at the CD-ring side chains were designed, synthesized, and evaluated for their biological properties on restoring bone mass in ovariectomized (OVX) rats with established osteopenia. The synthesis of compounds 3 and 4 utilized the Wittig-Horner coupling to build up the vitamin D conjugated triene system, followed by the introduction of the cycloketone fragments at the side chain, and subsequent α,α-difluorination of the ketone by the treatment of the derived silyl enol ether with Selectfluor, as the key synthetic steps. In comparison with the natural 1α,25-dihydroxyvitamin D₃ (calcitriol; 200 ng/kg/day), oral administration of compounds 3 and 4 at the dose of 25 ng/kg/day for 6 weeks led to much improved bone mass and bone density related parameters, while maintaining normal serum calcium and serum phosphorus levels. The immunohistochemistry results showed that both compounds remarkably decreased in osteoclast number and moderately decreased in osteoblast number on trabecular bone surface. Therefore, our findings suggested that compounds 3 and 4 successfully rescue bone loss by suppression on bone turnover in OVX rat models.

Design, synthesis and biological properties of seco-d-ring modified 1α,25-dihydroxyvitamin D3 analogues

As a continuation of our efforts directed to the structure-activity relationship studies of vitamin D compounds, we present in this paper the synthesis of new analogues of 1α,25-(OH)₂D₃ characterized by numerous structural modifications, especially a cleaved D ring. Total synthesis of the CD fragment required for the construction of the target vitamins was based on the Stork approach. The structure of the key intermediate - bicyclic hydroxy lactone - was established by crystallographic and electronic circular dichroism (ECD) spectral analysis. Following the attachment of the hydroxyalkyl side chain, the formed D-seco Grundmann ketone was subjected to Wittig-Horner coupling with the corresponding A-ring phosphine oxides providing two desired D-seco analogues of 19-nor-1α,25-(OH)₂D₃, one without a substituent at C-2 and the other possessing a 2-exomethylene group. Both compounds were biologically tested and the latter was found to be more active in in vitro tests. Despite so many structural changes introduced in its structure, the biological activity of the 2-methylene analogue approached that of the natural hormone. The synthesized D-seco vitamins, however, proved to be inactive on bone and intestine in vivo.

Vitamin D receptor 2016: novel ligands and structural insights

INTRODUCTION

Vitamin D₃ activates via its hormonal form 1α,25-dihydroxyvitamin D₃ (1α,25(OH)₂D₃), the transcription factor vitamin D receptor (VDR). VDR is expressed in most human tissues and has more than 1,000 target genes. Thus, 1α,25(OH)₂D₃ and its synthetic analogs have a broad physiological impact. The crystal structures of the VDR ligand-binding domain (LBD), and its various ligands, allows further the understanding of the receptor's molecular actions. Areas covered: We discuss the most important novel VDR ligands and the further insight derived from new structural information on VDR. Expert opinion: There is an increasing appreciation of the impact of vitamin D and its receptor VDR not only in bone biology, but also for metabolic diseases, immunological disorders, and cancer. Detailed structural analysis of the interaction of additional novel ligands with VDR highlight helices 6 and 7 of the LBD as being most critical for stabilizing the receptor for an efficient interaction with co-activator proteins, i.e. for efficient agonistic action. This permits the design of even more effective VDR agonists. In addition, chemists took more liberty in replacing major parts of the 1α,25(OH)₂D₃ molecule, such as the A- and CD-rings or the side chain, with significantly different structures, such as carboranes, and still obtained functional VDR agonists.

Synthesis of 1α,25-dihydroxyvitamin D3 analogues with α-hydroxyalkyl substituents at C12

Convergent syntheses of three new analogues of 1α,25-dihydroxyvitamin D3 with α-hydroxyalkyl substituents at C12 (4a-c) are described. The A-ring and triene system of each analogue were assembled by a tandem Pd-catalysed intramolecular cyclization and Suzuki-Miyaura coupling process. The stereoselective introduction of substituents at C12 was achieved by Johnson-Claisen rearrangement on allylic alcohol 15 as the key step. This article is part of a Special Issue entitled 'Vitamin D Workshop'.

Vitamin D receptor ligands: the impact of crystal structures

INTRODUCTION

In the past years, the biologically active form of vitamin D(3), 1α,25-dihydroxyvitamin D(3) (1α,25(OH)(2)D(3)), has received large appreciation due to the broad physiological impact of the hormone and its nuclear receptor, the transcription factor vitamin D receptor (VDR). Recently, the understanding of VDR actions has progressed greatly, due to VDR crystal structures with various ligands.

AREAS COVERED

This review will present and discuss new synthetic agonistic and antagonistic 1α,25(OH)(2)D(3) analogs in the context of the recent insights provided by VDR crystal structures.

EXPERT OPINION

During the last 5 years, a large number of new 1α,25(OH)(2)D(3) analogs, many of which have an interesting functional profile, have been patented. Moreover, for a surprisingly high number of 1α,25(OH)(2)D(3) analogs, the crystal structure data of their complex with the VDR is available. This structural information provides important insight into the functional potential of the VDR ligands and explains their agonistic and antagonistic action. However, so far, only for a few VDR ligands, a rational design, based on crystal structure information, has been applied. The design of future analogs may also take the specificity of co-factor interaction into account, in order to create selective VDR modulators.

Design, synthesis, evaluation, and structure of vitamin D analogues with furan side chains

Based on the crystal structures of human vitamin D receptor (hVDR) bound to 1α,25-dihydroxy-vitamin D(3) (1,25 D) and superagonist ligands, we previously designed new superagonist ligands with a tetrahydrofuran ring at the side chain that optimize the aliphatic side-chain conformation through an entropy benefit. Following a similar strategy, four novel vitamin D analogues with aromatic furan side chains (3a, 3b, 4a, 4b) have now been developed. The triene system has been constructed by an efficient stereoselective intramolecular cyclization of an enol triflate (A-ring precursor) followed by a Suzuki-Miyaura coupling of the resulting intermediate with an alkenyl boronic ester (CD-side chain, upper fragment). The furan side chains have been constructed by gold chemistry. These analogues exhibit significant pro-differentiation effects and transactivation potency. The crystal structure of 3a in a complex with the ligand-binding domain of hVDR revealed that the side-chain furanic ring adopts two conformations.

Cyclical regulation of the insulin-like growth factor binding protein 3 gene in response to 1alpha,25-dihydroxyvitamin D3

The nuclear receptor vitamin D receptor (VDR) is known to associate with two vitamin D response element (VDRE) containing chromatin regions of the insulin-like growth factor binding protein 3 (IGFBP3) gene. In non-malignant MCF-10A human mammary cells, we show that the natural VDR ligand 1α,25-dihydroxyvitamin D(3) (1α,25(OH)(2)D(3)) causes cyclical IGFBP3 mRNA accumulation with a periodicity of 60 min, while in the presence of the potent VDR agonist Gemini the mRNA is continuously accumulated. Accordingly, VDR also showed cyclical ligand-dependent association with the chromatin regions of both VDREs. Histone deacetylases (HDACs) play an important role both in VDR signalling and in transcriptional cycling. From the 11 HDAC gene family members, only HDAC4 and HDAC6 are up-regulated in a cyclical fashion in response to 1α,25(OH)(2)D(3), while even these two genes do not respond to Gemini. Interestingly, HDAC4 and HDAC6 proteins show cyclical VDR ligand-induced association with both VDRE regions of the IGFBP3 gene, which coincides with histone H4 deacetylation on these regions. Moreover, combined silencing of HDAC4 and HDAC6 abolishes the cycling of the IGFBP3 gene. We assume that due to more efficient VDR interaction, Gemini induces longer lasting chromatin activation and therefore no transcriptional cycling but monotonically increasing IGFBP3 mRNA. In conclusion, 1α,25(OH)(2)D(3) regulates IGFBP3 transcription through short-term cyclical association of VDR, HDAC4 and HDAC6 to both VDRE-containing chromatin regions.

3D-QSAR Studies on C24-Monoalkylated Vitamin D3 26,23-Lactones and their C2α-Modified Derivatives with Inhibitory Activity to Vitamin D Receptor

The ligand-based three-dimensional quantitative structure-activity relationship (3D-QSAR) for 82 inhibitors of 25-dehydro-1α-hydroxyvitamin D3 -26,23-lactone analogs has been studied by using comparative molecular field analysis (CoMFA) and comparative molecular similarity indices analysis (CoMSIA) models. The established CoMFA model in training set gives a cross-validated q(2) value of 0.516 and a non-cross-validated rncv (2) value of 0.667, while the CoMSIA model results in q(2) =0.517 and rncv (2) =0.632. In general, the predictive ability of the CoMFA model is superior to that of the CoMSIA model, with rpred (2) =0.639 for the CoMFA and rpred (2) =0.619 for the CoMSIA model. Based on the CoMFA contour maps, some key structural characters of vitamin D3 analogs responsible for inhibitory activity are identified, and some new C2α-modified 24-alkylvitamin D3 lactone analogs with high predicted pIC50 values are designed. The ligand functional group mutations by FEP simulation and docking studies reveal the rationality of the molecular design.

A short practical approach to 24R,25-dihydroxyvitamin D3

A synthesis of the vitamin D3 metabolite 24R,25-dihydroxyvitamin D3 (1) by Lythgoe's Wittig-Horner approach is described. The key step of the synthesis is the stereocontrolled introduction of the 24-hydroxyl group by a palladium(0)-induced [3,3]-sigmatropic rearrangement on a 22R-allylic acetate (7).

Design and synthesis of active vitamin D analogs

A review of the design and synthesis of structural analogs of the vitamin D hormone recently investigated in our laboratories, and the first report on a new class of vitamin D analogs characterized by an aromatic D-ring, is described.

Stereoselective synthesis of C24-hydroxylated vitamin D3 analogs: a practical and expeditius route to calcipotriol

The synthesis of the clinically important drug calcipotriol (2, MC903) is described as an example of a new and efficient approach to C24-hydroxylated analogs and metabolites of vitamin D3 (1). The key step of the process is the generation of the C24 stereocenter by DAIB [(-)-3-exo-(dimethylamino)isoborneol]-catalyzed addition of the alkenylzinc derivative of alkyne 3 to cyclopropylcarboxaldehyde.

Therapeutic applications for novel non-hypercalcemic vitamin D receptor ligands

BACKGROUND

The active form of vitamin D(3), 1alpha,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)), plays an important role in calcium homeostasis, cell differentiation, cell proliferation and immunity. A more complete understanding of the several physiological and pharmacological properties of 1,25(OH)(2)D(3) indicates that the vitamin D receptor (VDR) is a promising drug target in the treatment of cancers, autoimmune diseases, infections and cardiovascular disease as well as bone and mineral disorders. The calcemic effect of 1,25(OH)(2)D(3) and its derivatives has limited their clinical application. As a result, the development of non-calcemic VDR ligands is required to realize the potential of VDR-targeting therapy.

OBJECTIVE

In this review, we discuss the in vitro and in vivo pharmacological actions, including VDR interaction, regulation of cofactor recruitment, pharmacokinetics and cell type or tissue-selective action of VDR ligands with less-calcemic activity.

CONCLUSION

Pharmacokinetic parameters and selective tissue accumulation are related to the therapeutic benefit of non-hypercalcemic vitamin D derivatives. Induction of distinct VDR conformations and cofactor recruitment may be associated with selective actions of non-secosteroidal VDR ligands. Derivatives of lithocholic acid, a newly identified endogenous VDR ligand, are less-calcemic VDR ligands.

Crystal structure of the vitamin D nuclear receptor ligand binding domain in complex with a locked side chain analog of calcitriol

The crystal structures of vitamin D nuclear receptor (VDR) have revealed that all compounds are anchored by the same residues to the ligand binding pocket (LBP). Based on this observation, a synthetic analog with a locked side chain (21-nor-calcitriol-20(22),23-diyne) has been synthesized in order to gain in entropy energy with a predefined active side chain conformation. The crystal structure of VDR LBD bound to this locked side chain analogue while confirming the docking provides a structural basis for the activity of this compound.

Synthesis and Conformational Analysis of 17α,21-Cyclo-22-Unsaturated Analogues of Calcitriol

Six new calcitriol analogues, conformationally restricted at their side chain by the introduction of both a cyclopropane ring at C17-C20 and a double or triple bond at C22, were synthesized using the Wittig-Horner approach to construct the triene system. The six CD-ring and side-chain bearing fragments were prepared from ketone 14 by a divergent route to generate both series of epimers at C20, followed by stereoselective cyclopropanation. The (E)-alkenyl side chain was synthesized by means of a Wittig reaction. The alkynyl side chain was prepared by Corey-Fuchs homologation, followed by alkylation. The (Z)-alkenyl side chain was prepared from the previous alkyne by partial hydrogenation. The 20-epi analogues bind more strongly to VDR than the corresponding analogues with the C20 natural stereochemistry. These results can be reasoned by conformational analysis and hydrophobic interactions with the VDR ligand-binding domain.

Synthesis of two carboxylic haptens for raising antibodies to 25-hydroxyvitamin D3 and 1alpha,25-dihydroxyvitamin D3

Hapten derivatives of 25-hydroxyvitamin D(3) and 1alpha,25-dihydroxyvitamin D(3) were synthesized using the Wittig-Horner approach. Both haptens bearing a carboxylic group at the side chain that can be linked to a protein for raising antibodies of potential utility for the determination of 25-hydroxyvitamin D(3), 1alpha,25-dihydroxyvitamin D(3) and 1alpha-hydroxylated vitamin D(3) analogues.

Novel 1α,25-Dihydroxyvitamin D3 Analogues with the Side Chain at C12

The plethora of actions of 1alpha,25(OH)2D3 in various systems suggested wide clinical applications of vitamin D nuclear receptor (VDR) ligands in treatments of inflammation, dermatological indication, osteoporosis, cancers, and autoimmune diseases. More than 3000 vitamin D analogues have been synthesized in order to reduce the calcemic side effects while maintaining the transactivation potency of the natural ligand. In light of the crystal structures of the vitamin D nuclear receptor (VDR), novel analogues of the hormone 1alpha,25(OH)2D3 with side chains attached to C-12 were synthesized via the convergent Wittig-Horner approach. Among the compounds studied, the analogue 2b showed the highest binding affinity for VDR and was the most potent at inducing VDR transcriptional activity in a transient transfection assay (20% of the transactivation activity of the natural ligand).

Vitamin D receptor agonists specifically modulate the volume of the ligand-binding pocket

Existing crystal structure data has indicated that 1alpha,25-dihydroxyvitamin D(3) (1alpha,25(OH)(2) D(3)) and its analogues bind the ligand-binding pocket (LBP) of the human vitamin D receptor in a very similar fashion. Because docking of a ligand into the LBP is a more flexible process than crystallography can monitor, we analyzed 1alpha,25(OH)(2)D(3), its 20-epi derivative MC1288, the two side-chain analogues Gemini and Ro43-83582 (a hexafluoro-derivative) by molecular dynamics simulations in a complex with the vitamin D receptor ligand-binding domain and a co-activator peptide. Superimposition of the structures showed that the side chain of MC1288, the first side chain of the conformation II of Gemini, the second side chain of Ro43-83582 in conformation I and the first side chain of Ro43-83582 in conformation II take the same agonistic position as the side chain of 1alpha,25(OH)(2)D(3). Compared with the LBP of the natural hormone MC1288 reduced the volume by 17%, and Gemini expanded it by 19%. The shrinking of the LBP of MC1288 and its expansion to accommodate the second side chain of Gemini or Ro43-83582 is the combined result of minor movements of more than 30 residues and major movements of a few critical amino acids. The agonist-selective recognition of anchoring OH groups by the conformational flexible residues Ala-303, Leu-309, and His-397 was confirmed by in vitro assays. In summary, variations in the volume of agonists lead to adaptations in the volume of the LBP and alternative contacts of anchoring OH-groups.

Development and application of diazirines in biological and synthetic macromolecular systems

Many different reagents and methodologies have been utilised for the modification of synthetic and biological macromolecular systems. In addition, an area of intense research at present is the construction of hybrid biosynthetic polymers, comprised of biologically active species immobilised or complexed with synthetic polymers. One of the most useful and widely applicable techniques available for functionalisation of macromolecular systems involves indiscriminate carbene insertion processes. The highly reactive and non-specific nature of carbenes has enabled a multitude of macromolecular structures to be functionalised without the need for specialised reagents or additives. The use of diazirines as stable carbene precursors has increased dramatically over the past twenty years and these reagents are fast becoming the most popular photophors for photoaffinity labelling and biological applications in which covalent modification of macromolecular structures is the basis to understanding structure-activity relationships. This review reports the synthesis and application of a diverse range of diazirines in macromolecular systems.

Gene Regulatory Potential of Nonsteroidal Vitamin D Receptor Ligands

The seco-steroid 1α,25-dihydroxyvitamin D3 [1α,25(OH)2D3] is a promising drug candidate due to its pleiotropic function including the regulation of calcium homeostasis, bone mineralization and cellular proliferation, differentiation, and apoptosis. We report here a novel class of nonsteroidal compounds, represented by the bis-aromatic molecules CD4409, CD4420, and CD4528, as ligands of the 1α,25(OH)2D3 receptor (VDR). Taking the known diphenylmethane derivative LG190178 as a reference, this study provides molecular evaluation of the interaction of nonsteroidal ligands with the VDR. All four nonsteroidal compounds were shown to induce VDR-retinoid X receptor heterodimer complex formation on a 1α,25(OH)2D3 response element, stabilize the agonistic conformation of the VDR ligand-binding domain, enable the interaction of VDR with coactivator proteins and contact with their three hydroxyl groups the same residues within the ligand-binding pocket of the VDR as 1α,25(OH)2D3. Molecular dynamics simulations demonstrated that all four nonsteroidal ligands take a shape within the ligand-binding pocket of the VDR that is very similar to that of the natural ligand. CD4528 is mimicking the natural hormone best and was found to be in vitro at least five times more potent than LG190178. In living cells, CD4528 was only two times less potent than 1α,25(OH)2D3 and induced mRNA expression of the VDR target gene CYP24 in a comparable fashion. At a noncalcemic dose of 150 μg/kg, CD4528 showed in vivo a clear induction of CYP24 expression and therefore may be used as a lead compound for the development of therapeutics against psoriasis, osteoporosis, and cancer.

SNAIL vs vitamin D receptor expression in colon cancer: therapeutics implications

Vitamin D analogues with reduced hypercalcemic activity are under clinical investigation for use against colon cancer and other neoplasias. However, only a subset of patients responds to this therapy, most probably due to loss of vitamin D receptor (VDR) expression during tumour progression. Recent data show that SNAIL transcription factor represses VDR expression, and thus abolishes the antiproliferative and prodifferentiation effects of VDR ligands in cultured cancer cells and their antitumour action in xenografted mice. Accordingly, upregulation of SNAIL in human colon tumours associates with downregulation of VDR. These findings suggest that SNAIL may be associated with loss of responsiveness to vitamin D analogues and may thus be used as an indicator of patients who are unlikely to respond to this therapy.

A structural basis for the species-specific antagonism of 26,23-lactones on vitamin D signaling

The 26,23-lactone derivative of 1alpha,25-dihydroxyvitamin D3, TEI-9647, is a partial antagonist of the of human vitamin D receptor (VDR). However, we found that TEI-9647 in rat cells behaves as a weak VDR agonist. This behavior could be mimicked in human cells by the double mutagenesis of human VDR (specifically C403S and C410N). The increased agonistic action of TEI-9647 correlates to a gain in the interaction of the VDR with coactivator protein and a decreased stabilization of the antagonistic conformation of the receptor. Molecular dynamics simulations indicated that TEI-9647 acts as antagonist of human VDR by reducing the stability of helix 12 of the ligand binding domain. In contrast, N410 of the rat VDR stabilized, via backbone contacts, the interaction between helices 11 and 12. This results in TEI-9647 becoming a weak agonist in this organism.